FR2758221A1 - DEVICE FOR FILTERING CARDIAC ACTIVITY SIGNALS - Google Patents

Abstract

This device receives as input signals originating from physiological data collection means, and delivers at output, to means for processing these data, signals extending, in the frequency domain, over a widened spectral band. The collection means comprise a first high-pass filter reducing the extent of the spectral band of the signal received at the input. According to the invention, the device comprises a compensation stage comprising a device for accentuating the frequency characteristic (32) reversed with respect to that (24) of the first high-pass filter, the cut-off frequency (f1) of the first pass filter -high being chosen higher than the lower frequency (f0) of the spectral band. Advantageously, it further comprises a second high-pass filter, the characteristic of which (38) has a cut-off frequency corresponding to the lower frequency (f0) of the spectral band.

Description

The present invention relates to the processing of the signals collected by

  medical devices, in particular "active implantable medical devices" as defined by Directive 90/385 / EEC of 20 June 1990 of the Council of the European Communities, for example cardiac pacemakers, defibrillators and / or cardiovers,

  neurological devices, drug delivery pumps

cochlear implants and implants.

  The invention will be mainly described in the case of a stimulation

  However, this is only an example of an implementation of the invention, which is applicable much more generally to a very wide variety of active, implantable or

  even non-implantable, for example worn by the patient.

  In these devices, cardiac activity is collected

  electrodes and the signal obtained is applied to an amplification module.

  and filtering. The amplifier is usually designed to receive

  see signals of the order of millivolt in a frequency band

  extending over a range typically from 1 Hz to 80 Hz.

  But the latest devices are pushing those limits.

  Indeed, for example, the VDD type stimulators exhibit

  much lower sensitivities, of the order of 0.1 mV, because they use

  In the atrium there is a floating electrode for collecting the P waves, and to capture these signals it is necessary to increase the gain of the amplifiers. In addition, current pacemakers have functions called

  "Holter", that is, memorization and analysis of cardiac activity.

  only over a very long time, typically over several hours.

  The analysis of the endocavitary signal that is operated for this purpose requires

  a bandwidth whose minimum frequency is much lower-

  is, typically 0.1 Hz, to be able for example to analyze the segment

ST of the heart signal.

  These improvements lead to the emergence of new problems.

  especially because of the much wider bandwidth

  in the field of low frequencies.

  First of all, regardless of its sensitivity, the amplifier

  should be able to withstand the high amplitude of the stimulus pulse.

  mulation likely to be applied by the device, an impulse

  the amplitude can reach 10 V, and then recover the fastest

  possible listening capacity (sensitivity) of signals of the order

millivolt.

  In order to be able to withstand the high voltage of the stimulating pulse, there is always a blanking-out period for the input circuits at the moment the pulse is delivered. However, at the end of the blanking, when the input amplifier is again switched on listening, a significant saturation of the input stage can still occur because the potential of the heart / electrode interface n ' is

  not yet returned to its resting value.

  This problem is further aggravated by the fact that

  the longer the cutoff frequency of the high-pass filter of the input stage is lower: thus, for a high-pass filter at 0.1 Hz cutoff, the time recovery

  the amplifier is of the order of 10 s, which is totally incompa-

  tible with the need to respond quickly to the succes-

cardiac signals.

  It is certainly possible to apply the so-called "pre- or post-

  charge "of delivering electrical charges before or after the stimulating pulse to compensate for the increase in recovery time, but this method is a heavy consumer.

  of energy and can not be applied permanently without

  significantly increase the life of the implanted device.

  Another problem lies in the size of the capacitive and resistive components, which is all the more high as the cut-off frequency of the high-pass filter of the input stage is lower. It is thus seen that the wider the listening window in the low frequency domain, the greater the need for large components, which is incompatible with the requirements of miniaturization of the circuits of the

implantable devices.

  One of the aims of the invention is to overcome these drawbacks, by proposing a device which makes it possible to significantly widen the spectral analysis band in the low frequency domain, without

  require large components and, above all,

  significantly the recovery time of the stimulator input amplifier. As will be understood, this problem is not peculiar to pacemakers alone, and arises each time that one is in the presence of signals collected by sensors such as activity sensor, respiration, etc.. which one wishes to be able to exploit later the very low frequency component while having for the treatment

  classic signal response times reduced.

  Essentially, the invention proposes to choose a cut-off frequency of the higher pass filter of the input stage of the stimulator higher than the usual values (for example 4 Hz instead of 1 Hz) and

  to provide a compensation stage, preferably with digital filtering.

  that, which offsets the attenuation due to the input filter.

  The compensation stage can only be used when it is desired to exploit the very low frequency components of the signal. It can either be integrated with the implantable prosthesis (to allow, for example, a real-time analysis of the cardiac signal) or incorporated for example into a programmer or other external device intended to cooperate with the implantable prosthesis.

  prosthesis implanted to treat the collected signals.

  More specifically, the device of the invention is a filtering device which is of the known type, receiving input signals of physiological data collection means, and outputting, to means for processing these data, signals extending, in the frequency domain, over an enlarged spectral band, the collection means comprising a first high-pass filter reducing the range of

  the spectral band of the signal received at the input. According to the invention, it

  takes a compensation stage with an accentuation device

  reverse frequency characteristic with respect to that of the first high-pass filter, the cut-off frequency of the first high-pass filter being chosen to be greater than the lower frequency of the

enlarged spectral band.

  According to a certain number of advantageous characteristics: the device furthermore comprises a second high-pass filter, whose characteristic has a cut-off frequency corresponding to the lower frequency of the enlarged spectral band; the accentuation device and the first filter passes. -have same cutoff frequency and inverse transfer functions; the order of the transfer function of the second high-pass filter is greater than the order of the transfer function of the first high-pass filter; the lower frequency of the enlarged spectral band is of the order of 0.1 Hz and the cutoff frequency of the first high-pass filter

is of the order of 4 Hz.

  The invention is equally applicable, symmetrically, to the extension

  frequency band up.

  This makes it possible, in particular, to restrict the band

  on the amplification and digitization stages by limiting the

  the influence of high frequency external signals

  generate saturations of the amplification stages, but in resto-

  later the useful signal in the whole width of the band

  spectral to allow a complete analysis.

  In this last form of implementation, the means for collecting

  include a first low-pass filter reducing the extent of the

  of the signal received at the input and, according to the invention, the device

  comprises a compensation stage comprising an amplification device

  inverted frequency characteristic tuition with respect to that of the first low pass filter, the cutoff frequency of the first low pass filter being chosen lower than the lower frequency of the band

enlarged spectral.

  In this case, according to a number of

  geuses: - the device further comprises a second low-pass filter, whose characteristic has a cutoff frequency corresponding to the upper frequency of the enlarged spectral band; the accenting device and the first low-pass filter have the same cut-off frequency and inverse transfer functions;

  the order of the transfer function of the second low-pass filter is su-

  less than the order of the transfer function of the first pass filter

  low. The filtering device of the invention, in one or other of its

  embodiments, can be incorporated into an impending medical device.

  active plantable, to an external programmer intended to cooperate with

  implanted active medical device, or even an

to such a programmer.

  Other features of the invention will become apparent on reading

  the description of an example of implementation below, with reference

attached drawings.

  Figure 1 is a block diagram of the chain of collection and

treatment of the cardiac signal.

  Figure 2 is a Bode diagram showing the characteristic

  attenuation of the filter of the input stage of the implanted device.

  Fig. 3 is a Bode diagram showing, respectively, the characteristic of the input stage filter and the characteristics of the

filters according to the invention.

  Figure 4 is a Bode diagram showing the characteristic

resulting spectral response.

  Figures 5, 6 and 7 are timing diagrams showing the pace of the endocavitary cardiac signal, respectively as collected, filtered by the stimulator input stage and reconstructed after application of

compensation according to the invention.

  FIGS. 8, 9 and 10 are similar to FIGS. 2, 3 and 4, when

  it is desired to broaden the spectral band in the field of high frequencies.

quences. 0o

  FIG. 1 is a block diagram of an implantable implantable device

  tif, comprising an electrode 10 collecting an endocardial

  cavity S1, which is applied to a circuit 12 for amplification and filtering.

  then to an analogue / digital converter.

  must have the necessary resolution to code the useful information of the

  signal S1 after it has been attenuated by block 12. The amplified signal

  filtered, filtered and digitized S2 feeds a plurality of modules, namely a

  detection module 14, a capture module 16 and a circuit 18 for analyzing

  lysis of the electrocardiogram (ECG) filtered by the circuit 12.

  These modules may be specific circuits or, alternatively, software blocks operating on a previously sampled and digitized signal within block 12.

  As indicated at the beginning of the description, the circuits

  such as circuit 18 for filtered ECG analysis operate on a

  spectral band typically ranging from 1 Hz to 80 Hz, these two frequencies

  quences corresponding to high and low pass cut-off frequencies

  below, respectively, of the filtering performed in block 12.

  As indicated at the beginning of this description, the cir-

  However, current stimulators are not suitable for

  the latest equipment.

  Indeed, to perform Holter functions, it is necessary to

  ability to analyze ECGs in a listening window starting at 0.1 Hz.

  But if one chooses for the input high-pass filter a frequency of

  much lower cutoff, the recovery time of the amplifica-

  becomes prohibitive, which creates an incompatibility with the need to respond quickly to the succession of cardiac signals. In addition, a high-pass filter with a very low cut-off frequency requires large capacitive and resistive components, which can

  not be integrable into an implantable device.

  To remedy this difficulty, the invention proposes to raise the

  cutoff frequency of the high-pass filter of block 12, for example -

  but of course not limiting - from 1 to 4 Hz, and to apply to signal S2 a corresponding inverse compensation (block 20), to deliver a signal S3, or "reconstructed signal" extending over a window

  wide range of listening in the field of low frequencies, typically

  up to 0.1 Hz - this value is of course not limiting.

  The signal S3 can be analyzed by an appropriate module (block 22), as

  for example an ST segment analysis that requires a band

  passing ECG signal starting frequency at 0.1 Hz.

  In a first embodiment, modules 20 and 22

  incorporated in the implanted device so as to enable

  in real time, and by the device itself, the intracavitary tracing.

  In another embodiment, the modules 20 and 22 are located in an external programmer, for example in the form of routines specific to a cardiac signal analysis software, the digitized endocavity signal then being transmitted to the programmer by telemetry means. In either case, the compensation device 20, like the high-pass filter of the block 12, can be made in analog form by specific filtering circuits. But they can also, advantageously, be realized digitally, in the form

  software routines implemented by the first microprocessor

existing device.

  We will now explain in more detail how the

  block compensation device 20.

  FIG. 2 shows the response curve (Bode diagram) of block 12, whose characteristic 24 has a flat portion between two frequencies f1 and f2, with attenuation beyond f2.

  (low-pass function) and also an attenuation below f1 (function

high pass).

  The choice of frequency f1 is a compromise mainly

  size considerations in block 12, but it is not critical. Note however that this frequency f1, which is

  typically 4 Hz, is significantly higher than usual frequencies.

  cut-offs of high-pass filters of conventional devices,

  the order of 1 Hz, which reduces the size of the components.

  The frequency f2, which is in fact not concerned by the invention, is

  typically of the order of 80 Hz, and defines the upper bound of the

be listening to the input.

  The high-pass filter of block 12 is generally a filter of the first

  first order, giving for part 30 of the characteristic an attenuation

  -6 dB / octave (we will see below that we can generalize to

higher orders).

  In FIG. 3, this characteristic of FIG.

  gure 2, to which corresponding compensation will be applied

  32 and 38, to give the resulting bandwidth

44 of Figure 4.

  To compensate for the attenuation 30 with the 4 Hz cut-off point, compensation 32 is applied with a flat response 34 above f1 = 4 Hz, and a gain 36 below. Ideally, the two frequencies are identical and equal to f1, but in practice we try to match them as well as possible so as not to generate distortions in the reconstituted signal. A first-order amplifying function can be used, giving a slope of +6 dB / octave. The combination of the slopes and 36 gives, below f1, a flat response, thus making it possible to

  to restore the very low frequencies of the endocavitary signal.

  The spectral components that had been

  attenuated by the input high-pass filter of block 12.

  Compensation 32, however, leads to infinite amplification

  for a zero frequency, so that it is eminently unstable.

  To correct this instability, we add, in the compensation stage,

  20, a high-pass filter whose characteristic 38 has a

  flat response 40 above a frequency fo, for example fo = 0.1 Hz, and

  a progressive attenuation 42 below this frequency.

  This second filter has a function of stabilizing the com-

  thought for very low frequencies (less than 0.1 Hz). It can be a first order filter (slope of -6 dB / octave) but in

  this case the result of characteristics 32 and 38 would also amplify

  the continuous component of the signal, in particular the voltage of

  the residual offset at the output of block 12, before scanning by block 13. That is why, to avoid having a gain on the component

  continuous, a filter 38 of the second order is preferably used: the first

  first order to stabilize the compensation feature 32 in

  very low frequencies, and the second order to cancel the offset

duel of block 12.

  In the case of a second-order filter, the two poles of it can be confused (as in the illustrated case, where one has a constant slope of -12 dB / octave below f0) or distinct , the two poles

  having, as noted, separate roles.

  The solution can be generalized to higher order filters

  for a characteristic of n x - 6 db / octave (high pass filter of

  dre n), the characteristic 36 will have a slope of n x + 6 dB / octave (n-order compensation filter) and the characteristic 42 a slope of

  (n + 1) x -6 dB / octave (high-pass filter of order n + 1) or, more generally

  a slope of (n + k) x -6 dB / octave, with k> 1 (high pass filter

of order n + k).

  FIGS. 5 to 7 illustrate the effectiveness of the device of the invention

on an example ECG signal.

  FIG. 5 corresponds to the signal S1 on the catheter before application.

  high-pass filtering of block 12.

  FIG. 6 represents this same signal after filtering has been applied

  block 12, for a first-order high-pass filter

  10 Hz cut-off, ie the S2 signal: we can see

  that the ST segment (after the main QRS peak) is significantly

  distorted and can not be properly analyzed on the basis of this

gnal.

  FIG. 7 illustrates the signal S3, after application of the compensation

  According to the invention, if we compare S3 and S1, we see that the signal

  constructed is extremely close to the original signal, and can therefore be

  analyzed in all its aspects by the practitioner or by a software of

appropriate analysis.

  The invention, as explained above, is also applicable

  cable to the widening of the spectral band in the field of

high frequencies.

  Figures 8, 9 and 10 show the characteristics 24, 32, 38 and 44 of Figures 2, 3 and 4, considered in the field of

  high frequencies of the Bode diagram.

  Beyond a frequency f2, for example f2 = 80 Hz, the characteristic

  frequency 26 of the input stage (FIG. 8) has an attenuation

  30 'of -6 dB / octave or, more generally, n x -6 dB / octave,

  n being the order of the corresponding low-pass filter.

  FIG. 9 represents, at the high end of the spectrum, the characteristic 32 of the enhancement stage, which has, below f2, a flat portion 34 and beyond f2 an accentuation 36 'of +6 dB / octave, or , in the case

general, of n x + 6 dB / octave.

  The second low-pass filter has a characteristic 38 with a

  flat portion 40 and an attenuation 42 'beyond a frequency f3, choose

  is greater than f2 (for example a frequency f3 = 200 Hz), with a

  slope of -12 dB / octave (or, in the general case, of (n + 1) x -6 dB / octah-

  ve) or, more generally, a slope of (n + k) x -6 dB / octave, with

  k> 1 (high-pass filter of order n + k). This ensures, by

  to what has been explained above and by transposing the reasoning to high frequencies, the stability of the system and the non-amplification

  residual noise at the output of the block 12 beyond the frequency of

pure f3 '

  Figure 10 illustrates the resulting bandwidth 44 in the

  high frequencies, with a flat portion 46 up to the frequency

  quence f3 (typically, f3 = 200 Hz), making it possible to restore the com-

  significant spectral components located in particular in the 80-

  Hz, then a strong attenuation 50 'beyond f3, allowing to attenuate

  the nonsignificant components of the signal, and especially

the noise component.

Claims (13)

  1. A filtering device, receiving as input signals (S1) of physiological data collection means, and outputting, to means for processing these data, signals (S3) extending, in the frequency domain , on an enlarged spectral band, the   collection means comprising a first high-pass filter (12) which reduces   the range of the spectral band of the signal (S1) received at the input,   characterized in that it comprises a compensation stage   tion (20) comprising a frequency characteristic enhancement device   quential (32) inverted relative to that (24) of the first pass filter   high, the cutoff frequency (f1) of the first high-pass filter being chosen greater than the lower frequency (f0) of the spectral band expanded.   2. The device of claim 1, further comprising a sec-   a high-pass filter, the characteristic (38) of which has a frequency   quence corresponding to the lower frequency (fo) of the enlarged spectral band.   3. The device of claim 1, wherein the device for accessing   centuation and the first high-pass filter have the same frequency of   pure (f1) and inverse transfer functions.   4. The device of claim 3, wherein the order of the function   transfer rate of the second high-pass filter is greater than the order of   the transfer function of the first high-pass filter.   5. The device of claim 1, wherein the lower frequency   The upper (f0) of the enlarged spectral band is of the order of 0.1 Hz.   6. The device of claim 1, wherein the frequency of   cutoff (fi) of the first high-pass filter is of the order of 4 Hz.   7. A filtering device, receiving as input signals from physiological data collection means, and outputting, to means for processing these data, signals extending, in the frequency domain, over an enlarged spectral band. , the collection means comprising a first low-pass filter reducing the range of the spectral band of the signal (Si) received at the input,   characterized in that it comprises a compensation stage   with a frequency characteristic enhancement device   quential (32) inverted relative to that (24) of the first pass filter low,   the cutoff frequency (f2) of the first low-pass filter being selected   less than the lower frequency (f3) of the extended spectral band ogy.   8. The device of claim 7, further comprising a sec-   low-pass filter, the characteristic (38) of which has a frequency   this cut corresponding to the upper frequency (f3) of the band enlarged spectral.   The device of claim 7, wherein the emphasis device and the first low pass filter have the same frequency of   cutoff (f2) and inverse transfer functions.   The device of claim 8, wherein the order of the transfer function of the second low-pass filter is greater than the order   of the transfer function of the first low-pass filter.   11. The device of one of claims 1 to 10, incorporated in a   active implantable medical device.   12. The device of one of claims 1 to 10, incorporated in a   external programmer arranged to cooperate with a medical device implanted asset.   13. The device of one of claims 1 to 10, incorporated in a   interface box to an external programmer intended to cooperate   with an implanted active medical device.